1. Field of the Invention
The present invention is directed to a retaining wall system. More particularly, the present invention is directed to a retaining wall system comprised of a retaining wall and an outer face. A method of forming such a retaining wall system is also provided. The disclosed retaining wall system and method are particularly adapted for use with excavated slopes or embankments formed in ground formations containing cobbles, boulders, rock, weathered rock, or cemented soil. Existing retaining wall structures and methods are either incompatible with ground formations containing such hard materials or present costly, unattractive or undesirable alternatives.
2. Description of the Prior Art
Tied Back Walls
One method of supporting the sides of excavations is to use a tied back wall. A tied back wall utilizes a plurality of tiebacks. A tieback comprises a grouted anchor installed in the ground mass to secure a tendon which applies a force to the earth retaining wall. The tiebacks and retaining wall support the ground mass behind the wall.
In a tied back wall, the force necessary to support the side of an excavation is applied to the tieback by testing and prestressing it against piles installed vertically in the ground mass. These vertical piles are commonly known as soldier beams. This method consists of the following steps:
installing a plurality of soldier beams into the soil mass in the area of the retaining wall to be formed; PA1 excavating the ground in front of the wall from the top down in successive sections; PA1 installing wood or precast concrete lagging between the soldier beams, or alternatively placing pneumaticaily applied concrete or cast-in-place concrete to help support the exposed face of the excavated section of the ground; PA1 installing tiebacks into the soil mass; PA1 testing and prestressing the tiebacks against the soldier beams; PA1 repeating the above steps as needed; and PA1 pouring a final outer layer of concrete over the pilings, lagging and tiebacks to form a permanent tied back wall. PA1 excavating the ground from the top down in successive sections; PA1 applying a layer or layers of pneumaticaily applied concrete to help support the exposed face of the excavated section of the ground; PA1 positioning retaining elements along the exposed face of the ground; PA1 installing tiebacks through the retaining elements and into the ground mass; PA1 testing and prestressing the tiebacks against the retaining elements; and PA1 repeating the above steps as needed until the entire wall is constructed. PA1 excavating the ground from the top down in successive sections; PA1 applying a layer or layers of pneumatically applied concrete to help support the exposed face of the excavated section of the ground; PA1 installing a row of soil nails through the pneumatically applied concrete into the ground mass; and PA1 repeating the above steps as needed. PA1 excavating the ground from the top down in successive sections; PA1 applying a layer or layers of pneumatically applied concrete to help support the exposed face of the excavated section of the ground; PA1 installing soil nails through the pneumatically applied concrete and into the ground mass; PA1 positioning retaining elements along the exposed face of the wall; PA1 installing tiebacks through the retaining elements and into the ground mass; PA1 testing and prestressing the tiebacks against the retaining elements; and PA1 repeating the above steps as needed. PA1 excavating a soil wall from the top down in successive sections; PA1 applying a layer or layers of pneumaticaily applied concrete to help support the exposed face; PA1 installing soil nails through the pneumaticaily applied concrete layer or layers and into the soil mass; PA1 installing tiebacks through the pneumaticaily applied concrete layer or layers and into the soil mass; PA1 repeating the above steps as needed; PA1 attaching or pouring and curing an outer layer of concrete; and PA1 testing and prestressing the tiebacks against the outer layer of concrete. PA1 installing tiebacks (or soil nails) through an existing retaining wall and into the ground mass; PA1 testing and prestressing the tiebacks against the existing retaining wall; and PA1 pouring a concrete wall over the existing retaining wall and the exposed end of the tiebacks.
U.S. Pat. No. 4,561,804 shows one type of tied back wall. The exposed face of the cut is supported in part by vertical sheet piles and either timber lagging or a layer of pneumaticaily applied concrete. The soil is then removed in descending stages until further support becomes necessary. At this point, tiebacks are installed through the sheet piles and into the soil mass. The tiebacks are then secured, tested and prestressed against the sheet piles. Excavation continues to the subgrade while lagging and, if required, more tiebacks are installed. A final layer of poured concrete is provided to form the finished, permanent retaining wall.
As shown in U.S. Pat. No. 4,561,804, vertically disposed sheet piles are driven into the ground mass to support the cut in a tied back wall system. While this construction method may be effective in ground formations containing soft soil or soil-like material, it is entirely inapplicable to a ground formation containing cobbles, boulders, rock, weathered rock, or cemented soil. The installation of vertical piling is not practical in these applications because the piling cannot easily be driven into the rock-hard ground formation.
Tied Back Element Walls (TE Wall)
A tied back element wall (TE wall) similarly uses a plurality of tiebacks. In a TE wall, the force necessary to support the side of an excavation is applied to the tieback by testing and prestressing it against retaining elements disposed along the exposed face of the soil mass. This method consists of the following steps:
TE walls are sometimes advantageous in that they do not require the installation of soldier beams. However, a TE wall construction is often more costly because the use of tiebacks is not optimized. A TE wall frequently requires between ten and thirty percent more tiebacks than a tied back wall using soldier beams. The additional tiebacks are necessary because the pneumatically applied concrete will not provide adequate support for the exposed slope face across great distances.
Further, TE walls constructed in ground formations containing cobbles, boulders, rock, weathered rock, or cemented soils frequently have an irregular surface. This irregular surface results from the inability to excavate a neat vertical cut in these types of ground formations. An irregular wall surface makes the installation of a permanent wall facing difficult and costly. Indeed, it is very difficult to attach a permanent face to the numerous discrete elements of the TE wall.
Soil Nail Walls
Soil nailing is another method which is used to retain the ground formed adjacent to an excavated slope. Soil nailing methods use untensioned tendons in grout-filled holes drilled into the exposed face of the excavated section of the ground. Soil nailing is often preferred over the above-mentioned tied back walls because soldier beams and timber lagging are not required. Soil nailing is often preferred over the above-mentioned tied back element wall because it is somewhat easier to attach a permanent concrete face to a soil nailing wall. This technique is thus less costly. In a soil nail wall, an array of nearly horizontal reinforcement rods (soil nails) are installed in the ground mass as the excavation proceeds downwardly. A reinforced layer of pneumatically applied concrete is used to support the exposed face between the soil nails. This method consists of the following steps:
U.S. Pat. Nos. 3,638,435; 3,802,204 and Re. 28,977 are exemplary of such a soil nail system. Two layers of pneumatically applied concrete are used to form an outer skin for an exposed slope wall. Boreholes extend through the skin and into the ground adjacent the wall. The boreholes are filled with grout and a reinforcing rod (soil nail) is then installed in the hole before the grout sets to form a dowel. One end of the rod (soil nail) extends outwardly from the skin to facilitate securing the dowel to the outer wall. In one embodiment, the rod (soil nail) is secured to the shotcrete skin itself via a bearing plate and fastener. In a second embodiment, the area surrounding the exposed end of the rod (soil nail) is filled with shotcrete to form a wale beam. The end of the rod (soil nail) is then secured to this shotcrete wale beam with the bearing plate and fastener arrangement.
As shown by the above patents, a soil nail wall depends upon a large number of untensioned dowels and a thin shotcrete face to support the entire excavated cut. Soil nailing is an economical earth retaining method for temporary and permanent applications in ground formations only where the nail holes may be drilled easily. Similarly, permanent soil nail walls are economical to construct only in soft ground formations where a neat vertical excavation can be made. Otherwise, as discussed above with respect to TE walls, the irregular surface makes the installation of a permanent wall facing difficult and costly.
U.S. Pat. No. 5,002,436 describes a retaining wall system which is comprised of a soil nail wall which is faced with pre-cast concrete panels. The facing panels are connected to the exposed ends of the soil nails via an adjustable coupling means. Backfill is then placed in the space between the exposed face of the excavation and the facing panels. This configuration is disadvantageous in several respects. First, the facing panels must necessarily be placed upon one another, and thus the completed outer wall must be vertical. Second, the required panel to soil nail connection significantly increases the overall cost of the retaining wall system. Finally, the panel to soil nail connection is embedded in backfill material and thus prone to deterioration from exposure to the environment.
Tieback Element and Soil Nail Walls (TEN Walls)
The TEN wall is a combination of the above-described tied back element and soil nail methods. A TEN wall is made up of a plurality of tiebacks, retaining wall elements and soil nails. This method utilizes short soil nails and pneumatically applied concrete to temporarily support a section of the soil wall to a certain depth. At some point of the excavation, when further support becomes necessary to retain the ground, a row of tiebacks is added. The soil nails and concrete support the ground between tied back elements, and both soil nails and tiebacks support over all the excavated face. Thus, shorter soil nails and tiebacks are required for the completed structure than in a wall built with only soil nails or tiebacks. This method consists of the following steps:
In the above described TEN wall method, both the soil nails and the tiebacks form a part of the final retaining wall support structure. The ultimate strength of the retaining wall will thus depend entirely on the strength of the soil nails and tiebacks themselves. No further reinforcement to the retaining wall structure is provided. The TEN wall is further subject to the limitations of the TE wall and soil nailing methods in that an irregular wall surface is formed.
Prior Art VSL Retaining Wall Methods
Other earth retaining wall systems and methods exist in the prior art which do not use vertically drilled piles or soldier beams. One such prior art method is shown at page 9 (bottom right photograph) of the "Soil and Rock Anchors" brochure published by the VSL Corporation. This method uses tied back vertical beams of cast-in-place concrete as support for an excavated face. These vertical beams are cast-in-place along the excavated face as the excavation proceeds downwardly. Tiebacks are installed through the vertical beams, and reinforcing steel extends from these beams. A cast-in-place face is then applied to the retaining wall and tied to the steel reinforcements protruding from the vertical beams. Walls built with vertical beams in this manner are difficult to construct because the vertical beams have to be built in short segments as the excavation proceeds, resulting in numerous reinforcing bar splices. Vertical beams are more expensive to build than horizontal beams since the concrete has to be placed using a concrete pump or with a concrete bucket and a crane. Vertical beam walls will require the tiebacks to be installed as the excavation proceeds rather than at their optimum location. Vertical beam walls are applicable for locations where the ground will support itself during construction. Attachment of precast concrete wall panels to vertical beams constructed against an excavated face is difficult and expensive since the front face of the beams must be at the proper location in order to allow the precast to be attached. Such vertical beam walls are relatively high and require long precast panels which are heavy and difficult to ship and handle.
A second prior art retaining wall method which does not use vertically drilled piles or soldier beams is described at page 11 of the VSL Newsletter published by the VSL Corporation. With this method, an excavated cut is made in successive sections from the top down. Each section is fully faced with reinforced cast-in-place concrete panels. These concrete panels are then secured to the exposed slope face via tiebacks. This method of construction is undesirable for ground formations containing rock or rock-like material since the integrity of a completed section of panels is undermined when the excavation of a section therebelow is commenced. In addition, this system is extremely expensive because the entire slope face must first be covered with cast-in-place panels and an additional outer face applied subsequently.
Also shown at page 9 (top photograph) of the VSL Corporation brochure entitled "Soil and Rock Anchors" is a third prior art retaining wall construction method which does not use soldier beams. With this method, horizontal concrete tie-beams and tiebacks are used to support an excavation made in a rock formation. However, this particular configuration is suitable only where the ground formation between the tie-beams is capable of supporting itself or where the cut face may be flattened to achieve a stable face. This system does not teach the facing of the horizontal tie-beams, nor is vertical support provided.
Other Prior Art Retaining Wall Methods
Another prior art retaining wall method utilizes soil nails and a layer or layers of pneumatically applied concrete to temporarily support the ground behind the earth retaining wall. Tiebacks are also installed, and a permanent outer concrete wall is poured and cured or attached. This method consists of the following steps:
With this method, the tiebacks are tested and prestressed against the final outer layer of poured concrete. Because the tiebacks are not tested and prestressed until the final outer wall is poured and cured, the tiebacks either visibly protrude through the completed wall or the wall must be patched at each tieback location. In either case the wall is unattractive.
U.S. Pat. No. 4,911,582 discloses a further prior art method which utilizes a concrete replacement wall to strengthen, repair or replace an existing earth retaining wall. In this method, tiebacks are installed through the exposed face and into the soil mass. Preferably, the tiebacks are installed in an open area of the existing wall which is filled with concrete. The tiebacks are then tested and prestressed against an anchorhead assembly placed against the existing wall. Finally, an outer concrete wall is cast in place over the entire exposed face of the wall. The outer wall is reinforced with a grid of reinforcing bars, which is positioned before the concrete wall is cast. Alternatively, soil nails may be used instead of tiebacks. The method thus consists of the following steps:
The above-described method is specifically designed for shoring up an existing retaining wall, not a newly formed embankment. Thus, no excavation takes place and a retaining wall with an outer face is not formed. Rather, the method of U.S. Pat. No. 4,911,582 is directed solely to a concrete replacement wall which is designed to strengthen, repair or replace an existing earth retaining wall.
It can thus be seen that there is a need for an improved method of forming a retaining wall with an outer face. In particular, there is a need for a retaining wall system which may be formed in ground formations containing cobbles, boulders, rock, weathered rock, or cemented soil where the installation of vertical piling is impracticable. Such a retaining wall system must be capable of adequately supporting the excavated cut without the need for soldier beams. In addition, such a retaining wall system must be designed to overcome the disadvantages inherent in existing prior art retaining wall systems by providing a structurally sound retaining wall which is less costly to construct and which has an outer face which is ultimately pleasing to the eye.